1. Field of the Invention
This invention relates generally to the field of CO2 sequestration and more specifically to apparatus and methods for sequestering CO2 using algae.
2. Background of the Invention
Increasing global levels of carbon dioxide (CO2) has been a worldwide concern for some time. Measured in terms of volume, there were about 280 parts per million CO2 in air at the beginning of the Industrial Revolution, and today there are 360 parts per million (ppm), a thirty percent increase. The annual increase is 2 ppm, and rising. If present trends continue, the concentration of CO2 in the atmosphere will double to about 700 ppm in the latter half of the 21st century. Many scientists now believe that most of the global warming observed over the past 50 years can be attributed to this increase in carbon dioxide from human activities.
It is well known that green plants uptake CO2 through photosynthesis. Photosynthesis converts the renewable energy of sunlight into energy that living creatures can use. In the presence of chlorophyll, plants use sunlight to convert CO2 and water into carbohydrates that, directly or indirectly, supply almost all animal and human needs for food. Oxygen and some water are released as by-products of this process. The principal factors affecting the rate of photosynthesis are a favorable temperature, level of light intensity, and availability of carbon dioxide. Most green plants respond favorably to concentrations of CO2 well above current atmospheric levels.
While there are a number of ways to increase carbon dioxide uptake in biological systems such as plants, it has proven difficult to do so cost effectively. Various strains of algae offer the fastest CO2 uptake. Ocean based enrichment programs are invasive and may lead to more problems than they solve. Specifically they tend to grow weed and filamentous forms of algae and can damage or destroy entire ecosystems. Efficient methods of harvesting the algae produced by such means are not in advanced development.
Land-based algae systems are very effective in capturing CO2, but are limited by available land space and cost. In an open passive or batch system, it is only possible to produce approximately 150 metric tons of dry biomass from algae per hectare per year. Using these figures, it would require over 200 hectares (495 acres) of open land to capture the output from a 1000-megawatt gas turbine power plant, not even taking into consideration weather and water availability. Critical to the production of large amounts of algae is the presence of light. Algae use light to convert CO2 into sugars, i.e. photosynthesis. Unfortunately, light only penetrates a few centimeters into an active culture of algae. As the algae organisms multiply and the culture density increases, the degree of light penetration decreases. Some researchers have used fiber optics as a light source but thus far this method has been prohibitively expensive and ineffective. Consequently, there is a need for apparatus and methods for sequestering CO2 using algae, which exposes the algae to a sufficient amount of light in a cost-effective manner.